hmm I would have to say Forgotten Realms has always been my personal favourite, largely because it is detailed and very "atypical" fatasy while retaining enough basic similarities to our own cultures as to provide something "familiar" enough for anyone to play.
Spelljammer I really loved with a passion and it appeled to means as the planescape alternative to link all the settings.
Dragonlances Krynn I always loved the feel of but always found it lacking something none of us could ever put our finger on when running it.
Greyhawk was good but after the first boxed set never really got the attention it deserved and why it was continued once the Forgotten Realms kicked in i'm not sure as it never got treated in any worthwhile fashion, better to have given its license to someone else who could have given it the time and attention it deserved imo. Not keen on the "D20 standard campaign" nature of it atm as it isn't supported by the other books that get released, so it ends up as more of a non relevant hindrance than helpful greyhawk flavouring here or there..
Planescape never bothered much with it, it looked good but my group were a little too Prime Material for it to appeal.
Council of Wyrms nice idea, but didnt really go anywhere after the intial boxed set.
Dark Sun, personally I loved it it had a "feel" I really dug, was never overly keen on the above 1st level approach too much though as Im more of a grass routes start to a campaign DM myself liking the munchkins to go from 1st to 20th with no shortcuts. Only thing Dark Sun lacked imo was a decent set of psionic rules back then to back up the awesome setting.
Red Steel never looked at it so couldn't comment... in truth i dont think I even saw the boxed set for it in any shop until about 3 years ago LOL.
Birthright again I loved it love anything with scope to it and Anuire has it by the bucketload... I had a few issues with the domain rules, but nothing major. Biggest problem with this setting was my group as the whole Regent angle didn't appeal to them as they hate bookkeeping and aspects of boardgame that encroach on thier roleplay... they need smiting I know

Though fortunately most hopefully have grown out of thier small minded ways... but there hasn't been much in the way of Domain running rules for D20 to date.
Al Quadim.. was nice I liked it, but to be honest its just part of the forgotten realms as far as im concerned.
Mystara I liked traditional D&D, ran a campaign in it a few times, but I think my group not knowing the background of it didn't really get the charm of it so it went largely unappreciated by them.
Ravenloft, had some great adventures there, ran some enjoyable and memorable games there, my only gripe at it was its 1800's approach instead of traditional fantasy.. it made it more pulp and hammer house than typical fantasy. Still the van richtens books were pure gold for ideas.